Determination of the rate of flow of fluids is especially important in aviation. Air speed and Mach number (the ratio of airspeed to the speed of sound at flight level) are useful not only for navigational purposes and flight control, but also for engine operation. Modern high performance aircraft, for example, use local measurements of the Mach number of gases flowing through engine ducts to control the setting of engine performance parameters.
Conventional determination of compressible flow Mach number is based on a known relationship between Mach number and measured fluid impact and static pressures. The static or nonmoving ambient pressure of the fluid can be measured directly by a pressure sensor sheltered from the velocity head, such as by a closed-ended tube having circumferential perforations at right angles to the flow. The impact pressure or differential pressure exerted due to the relative movement between the fluid and an object is generally derived from a measurement of total pressure, which is the sum of impact pressure and static pressure. Total pressure is typically measured using a pitot tube that has an open end which is pointed directly into the flow. An example of a system using pitot-static tube measurements to determine the Mach number of gases flowing through a duct of an aircraft engine is given in Plett et al U.S. Pat. No. 3,717,038.
A system has been proposed, as shown in Hall U.S. Pat. No. 3,039,305, for acoustically determining aircraft Mach number by means of an open-ended tuned tube with a sound source positioned at one end and with the other end open and facing upstream against the moving airstream. Sound wave emission out of the tube is suppressed by the airstream flow which acts as a barrier or acoustical impedance to reflect the sound waves back into the tube. The acoustical impedance increases with increasing speed of airflow and loads the sound source to decrease the average sound power radiated per cycle. Mach number can then be determined from previously derived relationship between the power emitted and the Mach number of the air stream flowing into the tube.
Both the conventional pitot-static tube and the suggested Hall patent tube arrangements require facing a tube into the fluid flow. The tubes are thus exposed to clogging or breakage due to ice particles or other debris which can interfere with their proper functioning, or render them totally useless. Futhermore, the intrusive nature of such devices limits their possibilites for placement in confined regions of ducts, such as aircraft engine ducts.
Considerable research has been conducted into the acoustic impedance effects of resonating cavities to sound generated in fluid flows as a means to achieve aircraft noise reduction, and in particular the reduction of noise generated in the ducts of gas turbine engines. Examples of such research are described in Kooi and Sarin, "An Experimental Study of the Acoustic Impedance of Helmholtz Resonator Arrays Under a Turbulent Boundary Layer," AIAA 7th Aeroacoustics Conference, Palo Alto, CA, Oct. 5-7, 1981, Paper No. 81-1998; and in Fiske, Syed and Joshi, "Measurement of Acoustic Modes and Wall Impedance in a Turbofan Exhaust Duct," AIAA 8th Aeroacoustics Conference, Atlanta, GA, Apr. 11-13, 1983, Paper No. 83-0733.
In aircraft gas turbine engines, noise reduction devices in the form of Helmholtz resonator arrays have been placed on duct walls with resonator parameters designed to absorb acoustic energy at selected frequencies, thereby providing quieter engines. Predicted acoustic impedances of such resonators at various engine noise frequencies have been compared against measured impedances at various known Mach numbers. The developed relationships have not, prior to this invention, been applied for determining flow Mach number for measured acoustic effects.